1. Field of the Invention
The present invention relates to a fuel supply system, a marine propulsion device and an outboard motor.
2. Description of the Related Art
In general, a fuel supply system for an outboard motor includes a sub fuel tank that stores a fuel supplied from a main fuel tank, a booster pump disposed inside the sub fuel tank, a fuel pipe connected to both of the booster pump and a fuel injection device of an engine, and a pressure regulator that keeps constant the pressure of the fuel inside the fuel pipe. The pressure regulator returns an excess amount of the fuel inside the fuel pipe to the sub fuel tank through a return pipe in order to keep constant the pressure of the fuel inside the fuel pipe.
In this type of fuel supply system, the temperature of the fuel is increased due to radiation of heat from an electric motor that drives the booster pump, and bubbles are inevitably produced inside the fuel pipe. Especially when a small amount of the fuel is required to be injected in the engine, the excess amount of the fuel to be returned to the sub fuel tank increases. Hence, the temperature of the fuel is more likely to increase.
In light of this, for the purpose of reducing the excess amount of the fuel to be returned to the sub fuel tank, Japan Laid-open Patent Application Publication No. 2000-220548 discloses a method of reducing the discharge amount of the booster pump by controlling the duty cycle of the booster pump when a small amount of the fuel is required to be injected in the engine.
However, in the method described in Japan Laid-open Patent Application Publication No. 2000-220548, the excess amount of the fuel is similarly returned to the sub fuel tank through the return pipe. Hence, this method has a limitation in preventing an increase in the temperature of the fuel.